A liquid crystal panel used in a liquid crystal display device such as a liquid crystal television is not self-luminous. Therefore, a separate backlight unit is required as an illumination device. Such a backlight unit is usually installed behind of a liquid crystal panel (on a side opposite from a display screen). A known backlight unit includes a large number of linear light sources and an optical sheet such as a diffusing plate arranged on a light-outputting side of the linear light sources (see Patent Document 1).    Patent Document 1: Japanese Patent Laid-Open No. 2007-256385